1. Field of the Invention
The present invention relates to a method of manufacturing a silicon steel sheet having grains precisely arranged in the Goss orientation.
2. Description of the Related Art
Grain-oriented silicon steel sheets have better magnetic properties than non-oriented ones, and are mainly employed as the core material of transformers. After Goss's invention for the method of manufacturing a silicon steel sheet having all the crystal grains oriented in {110} &lt;001&gt; orientation, there have been proposed a number of methods of manufacturing a grain-oriented silicon steel sheet having a Goss texture. These proposed methods have been classified mainly into three categories as follows:
The first category is directed to the two-time cold press method. This method is a remodeled version of the Goss's method, and in the two-stage cold rolling method, Mn, Sb, S, Se, and the like are added to the material in the steel refining process, and the secondary recrystallization is carried out by utilizing the crystal grain growth inhibiting effects of these elements themselves, and the fine precipitates thereof. More specifically, a steel ingot having a composition of C: 0.02 to 0.08 wt %, Si: 2.0 to 4.0 wt %, Mn: about 0.2 wt %, and S: 0.005 to 0.05 wt %, is melted and subjected to a hot rolling process to make a sheet having a thickness of 2.0 to 3.0 mm. Then, the hot-rolled sheet is annealed, and subjected to a cold rolling process, the rolling reduction of which is about 70%. After that, again, intermediate annealing is carried out at a medium temperature in the range of 850.degree. to 1050.degree. C., and subjected to a cold rolling process of a rolling reduction of 60 to 70%. Further, after a decarburization-annealing process is performed at 800.degree. to 850.degree. C., an annealing process is again conducted at a temperature of 1100.degree. C. or higher for 5 to 50 hours for the secondary recrystallization and removal of the inhibitors (purification-annealing). Thus, Goss grains are grown (see for example, Published Examined Japanese Patent Application No. 51-13469).
The second category is directed to the one-stage cold rolling method. In this method, the cold rolling process is carried out once, and this method is known to produce a sheet having a better Goss texture rate than the two-time cold rolling method. More specifically, a steel ingot having a composition of C: 0.02 to 0.08 wt %, Si: 2.0 to 4.0 wt %, Mn: about 0.2 wt %, and N: 0.01 to 0.05 wt %, and Al: about 0.1 wt %, is melted and subjected to a hot rolling process to make a sheet having a thickness of 2.0 to 3.0 mm. Then, the hot-rolled sheet is annealed, and subjected to an AlN deposition process. Then, the sheet is subjected to a cold rolling process of a rolling reduction of 80 to 95%, and a decarburization-annealing process is performed. After some time, an annealing process is again conducted at a high temperature of 1200.degree. C. for 20 hours for the secondary recrystallization and removal of the inhibitors (purification-annealing). Thus, Goss grains are grown (see for example, Published Examined Japanese Patent Application (PEJPA) No. 40-15644).
The third category is directed to the method in which the Goss texture is created without using inhibitors (see for example, Published Unexamined Japanese Patent Applications (PUJPA) No. 64-55339 and No. 2-57635, etc.). In this method, a rolling process and a heat treatment are simply combined with each other under a particular condition to grow Goss grains.
As described, the decarburization-annealing and purification-annealing are essential to the methods of the first and second categories. Since these annealing processes are each performed at a high temperature and for a long period of time, it is impossible to keep the production cost and equipment cost low.
Further, if the final product, sheet is formed to have a thickness of 0.20 mm or less to reduce the iron loss, the secondary recrystallization becomes unstable, and thus it is difficult to occupy all the surfaces with Goss grains. With the latest technique, the minimum thickness of the sheet is about 0.23 mm.
The method of the third category does not require decarburization-annealing, or purification-annealing; therefore this method is more cost effective in production than those of the first and second categories. However, the inventors of the present invention conducted tests to verify the methods disclosed in PUJPAs No. 64-55339 and 2-57635, and found out that the Goss grain growth mechanism is unstable, and therefore materials having all surfaces covered with Goss grains are not always obtained. Thus, it is difficult to obtain a stable quality. It should be emphasized here that a stable Goss grain growth is practically essential to a grain-oriented silicon steel sheet. Even if the product sheet is used after removing the section of other than the Goss grains, the production cost becomes high due to a poor yield.